51 Pegasi is a pretty ordinary star. But in 1995, it became one of the best-known stars in the galaxy. That’s because it was the first “normal” star with a confirmed planet. The newfound world was a giant, like Jupiter, the largest planet in our own solar system. But it was orbiting much closer in than Mercury is to the Sun. So astronomers described the new world as a “hot Jupiter.”

Since then, astronomers have discovered scores of hot Jupiters orbiting other stars. And they’ve recently studied 63 of them that were discovered by the planet-hunting Kepler spacecraft.

The scientists looked for small deviations in the orbits of these planets caused by the gravitational tug of other planets in the same star systems. But they found that not a single hot Jupiter has a close planetary neighbor.

The result may shed light on just how hot Jupiters form. The planets probably are born far from their stars, just as Jupiter was, and are kicked inward by the gravity of another giant planet. As the hot Jupiter moves toward its star, its gravity hurls away any other planets that lie closer to the star. Some of these planets fall into their star, while others are ejected into the deep cold of interstellar space.

51 Pegasi is about a third of the way up the eastern sky at nightfall. It’s just above the top side of the Great Square of Pegasus, which is in fine view. 51 Pegasi is faint, so you need dark skies to see it — a star system with a hot but lonely planet.

Script by Ken Croswell, Copyright 2012

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